• We would like to remind our members that this is a privately owned, run and supported forum. You are here at the invitation and discretion of the owners. As such, rules and standards of conduct will be applied that help keep this forum functioning as the owners desire. These include, but are not limited to, removing content and even access to the forum.

    Please give yourself a refresher on the forum rules you agreed to follow when you signed up.

Axe-Fx III Firmware Release Version 19.00

Potential bug: When I play through either the Legend 100 I or II, these three speaker impedance curves cause the sound to muffle completely with a low frequency sound:

1x12 Dirty Shirley EV12L
4x12 Hipower
4x12 Rumble

I'm wrong. I'm just wrong. Here's the skinny:

So I looked into this. I'm pretty sure the real amp would do the same thing. What is happening is the power amp is oscillating at that very low frequency cabinet resonance. This occurs because the Legacy has so much negative feedback (12 dB, which is the highest of any amp I've ever modeled). Combined with the smallish output transformer there is a loss of phase margin at low frequencies which causes the power amp to become unstable.

The Rumble cab is unique in that it is ported and has two different types of speakers in it. This causes a double resonance and one of the resonances is at a very low frequency. For typical power amps with reasonable amounts of feedback this isn't an issue.

You can stabilize the power amp by reducing the Transformer LF. This will improve the phase margin and increase stability. Another solution is to reduce Cabinet Resonance but that will change the tone a bit.

Whenever you have negative feedback you run the risk of instability. Highpass and lowpass filters introduce phase shift which reduces phase margin. Combine that with a strong resonance and the system can become unstable.

What I found is that lowering the Transformer LF works with the Rumble 4x12, but with the Dirty Shirley, 1x12 USA and HiWatt, you need to raise it.
 

yyz67

Power User
What I found is that lowering the Transformer LF works with the Rumble 4x12, but with the Dirty Shirley, 1x12 USA and HiWatt, you need to raise it.
Amazing: works as designed, or as configured by selecting those SICs.

Would be interesting if anyone has experienced that kind of oscillation in a real amp accidentally or otherwise.
 

reclavea

Power User
What an update, yet again!

Things that might help to hear a difference when changing between different power tubes:
  • Preferably use an amp you have experience with, so you are used to how it sounds.
  • Play a riff or a lick you really know well. Easier to hear subtle differences that way. If you keep hitting just one note or a chord and keep switching back and forth the differences might not be that audible.
  • Sometimes the differences just are not that audible. Can be also a feel thing. That’s why hitting a chord and switching back and forth doesn’t work that well. Playing different stuff with variable dynamics might help you to spot the differences.
  • You can use a looper so you can focus more on the sound, but I personally think it’s also important to hear how the different tube types react on your playing while your at it.
If you can’t hear a difference, just leave it be 🙂

The subject has been discussed for years and someone posted a link earlier of a heavy discussion here.

Very informative!

I’m still of the opinion that when changing tubes on a real amp the impact on the response and sound can be very dramatic.

I posted before that with my Mesa Mark III & 2x12 cab with EVs the outer tubes are interchangeable between 6L6 or EL34.

Mesa described it as the “American Rock” flavor with 6L6 and the “British Rock” flavor with the EL34.

Both sounds were awesome with the 6L6 sounding more smooth and warm and the EL34 setup sounding more open, harsh and not as smooth.

The difference was dramatic.

I stayed with the 6L6. I prefer smoother leads.

This is probably why we lay techs are still discussing.

Again this was discussed heavily and still being discussed.

Maybe it’s something that needs to addressed in future advancement.

Then again ....maybe not.

I’m content with Fractal progressing as they see fit!

A herd of elephants cannot ever get me away from my Axe!!

The FOH sound is all that really matters for me at this point. (& a decent monitor)
 

666was999

Fractal Fanatic
When I play real amps I often notice how loud the SC hum is there, often way more prominent than in my presets. Could be that my presets are still too dark sounding, I don't know. However that hum from the guitar or noise from my wireless is what real amps amplify too besides the wanted signal. And if I want a perfect copy of the real sound, might be I also get exactly the same hum and noise.
So rise the threshold of the noisegate and see hum as something that belongs there, ok?
 
A few years back when I owned a Diezel Einstein 100w head, I experimented a lot with different tubes.
I could clearly hear the difference between different 12ax7 brands (EV vs JJ vs Sovtek,...)
Changing power tubes between 6l6, 6550, KT77, 6ca7 also made a noticeable difference.
In the Axe, I can't tell the difference. I wonder if this is a volume thing ?
The Diezel was usually set very loud.​
 

Thomas Obester

Experienced
A few years back when I owned a Diezel Einstein 100w head, I experimented a lot with different tubes.
I could clearly hear the difference between different 12ax7 brands (EV vs JJ vs Sovtek,...)
Changing power tubes between 6l6, 6550, KT77, 6ca7 also made a noticeable difference.
In the Axe, I can't tell the difference. I wonder if this is a volume thing ?
The Diezel was usually set very loud.​
I had VH4, VH2 and Herbert.
I also could hear clear difference when changing tubes in those amps.
Diezels are known for their sensitivity and quite a big change in sound/feel when you retube them.
On diezel forum it is explained that this is because of it´s unique circut design, IDK.
With fractal, I just laugh how small the difference is when changing preamp/power amp glass ... :D
I embrace fractal for what it does, read all the tech notes..but then, again, you pull out your amp, change tubes and there is pronounced change, even at low volume.
 
Last edited:

Immanuel

Inspired
A few years back when I owned a Diezel Einstein 100w head, I experimented a lot with different tubes.
I could clearly hear the difference between different 12ax7 brands (EV vs JJ vs Sovtek,...)
Changing power tubes between 6l6, 6550, KT77, 6ca7 also made a noticeable difference.
In the Axe, I can't tell the difference. I wonder if this is a volume thing ?
The Diezel was usually set very loud.​
If you up the master volume, you may hear more of a difference.
 

Dave Merrill

Axe-Master
I had VH4, VH2 and Herbert.
I also could hear clear difference when changing tubes in those amps.
Diezels are known for their sensitivity and quite a big change in sound/feel when you retube them.
On diezel forum it is explained that this is because of it´s unique circut design, IDK.
With fractal, I just laugh how small the difference is when changing preamp/power amp glass ... :D
I embrace fractal for what it does, read all the tech notes..but then, again, you pull out your amp, change tubes and there is pronounced change, even at low volume.
It's also possible that there's more random production variation in real-world tubes than in the carefully controlled models of them in the Axe. Some of the differences you heard may be down to that, not characteristics of specific tube types or brands.
 

Thomas Obester

Experienced
It's also possible that there's more random production variation in real-world tubes than in the carefully controlled models of them in the Axe. Some of the differences you heard may be down to that, not characteristics of specific tube types or brands.
could be.
OTOH, fractal modeling is evolving week by week.
We always have some measurements, where Cliff shows with his graphs, how things are accurate or there is next to no difference to the real counterparts, but then, later down the road we have updates with now things being even more accurate.
Don´t get me wrong, I love this progress :)
But maybe, just maybe, at this point, changing tubes in fractal may not give you the same results as in analog domain.
 
Last edited:
Top Bottom